The long-term goal of this project is to develop prosthetic foot technology that can function differently for different activities. Although current prostheses can be very effective for particular tasks (e.g. walking, running, or standing), an amputee must choose a single prosthesis to achieve reasonably good performance across all daily activities. This results in a compromise, because the biomechanical demands of walking, for example, are different from those for standing. A prosthetic foot that can modulate its mechanical properties to biomechanically accommodate each activity could be very helpful. This prosthesis is intended for use by subjects with amputation of both traumatic and non-traumatic origin. The proposed biomechanical behavior is to modulate foot stiffness differently for many activities, most notably walking and standing. During walking, the prosthesis needs to provide strong weight support to prevent hard landings on the intact leg. This behavior is facilitated by high stiffness. During standing, different users may desire a stable base of support or a compliant foot with large range of motion. The ideal stiffness for each task also depends on the differing demands of dynamic and static stability. Based on studies of gait mechanics, stiffness should be adjusted for each different task. The Specific Aims of this project are to: 1.To design and construct a prosthetic foot that modulates its mechanical stiffness under automatic computer control based on the user's activity, and test its mechanical characteristics such as stiffness, adjustability, and fast modulation. 2. To perform proof-of-concept testing of the prototype prosthetic foot's effect on walking and standing in amputees.